Most engines on motor vehicles with brake boosters using vacuum (brake booster vacuum) for brake pedal assist utilize the engine intake manifold as a vacuum source for the brake booster to provide sufficient brake booster vacuum to meet vehicle braking requirements.
Some engines now use Displacement on Demand, which deactivates cylinders when load requirements are low, and/or use variable valve timing and/or other engine controls that can affect brake booster vacuum. These technologies improve fuel economy by reducing intake manifold vacuum (that is, the absolute pressure of the intake manifold increases) in order to reduce pumping losses. When the reduced intake manifold vacuum is such that the brake booster vacuum exceeds a calibrated brake booster vacuum threshold (that is the absolute pressure of the brake booster vacuum is higher than the absolute pressure of the brake booster vacuum threshold), it could theoretically result in inadequate brake booster vacuum to meet vehicle braking requirements and engine operation must be modified to provide adequate intake manifold vacuum to meet brake booster vacuum requirements for vehicle braking. For example, on engines with Displacement on Demand, the engine operation is modified to disable cylinder deactivation to increase manifold vacuum, whereas on engines with variable valve timing, engine operation could be modified to increase intake valve opening to increase manifold vacuum.
Presently, a vacuum sensor could be used in the brake booster to determine if the brake booster vacuum exceeds a calibrated brake booster vacuum threshold, whereupon engine operation would be modified to increase manifold vacuum to meet brake booster vacuum requirements for vehicle braking, but at an added cost per vehicle. Another alternative could be a supplemental vacuum pump to assist manifold vacuum to meet brake booster vacuum requirements for vehicle braking, but at an even higher cost per vehicle.
Accordingly, what is needed in the art is a less costly alternative to provide adequate intake manifold vacuum to meet brake booster vacuum requirements for vehicle braking by modifying engine operation when the intake manifold vacuum is such that the brake booster vacuum exceeds a calibrated brake booster vacuum threshold.